Q&A: Painting PT Decking

Chimney Flashing for Metal Roofs

Q.Some of my customers want
their pressure-treated decks painted, but I’ve
had trouble getting paint to adhere well to
pressure-treated wood. What’s the
solution?

A.Mark Knaebe
responds: Treated lumber is not the source of
your deck-painting difficulties. Clean, dry
CCA-treated lumber is actually a better surface to
paint than untreated wood of the same species. The
real problem is that a paint’s performance
suffers on a horizontal surface that is exposed to
the weather. To make matters worse, deck boards are
usually flat-grained, high-density wood that
doesn’t hold paint as well as
edge-grained, low-density wood.

For exterior decks, you’re better off
using a water-repellent preservative or a
penetrating-type semitransparent pigmented stain.
Solid-color stains and film-forming paints
aren’t recommended for horizontal surfaces
because they may fail early. Hard enamel paints
lack the flexibility to accommodate the movement of
exposed wood. Flexible latex paints are not tough
enough to stand up to foot traffic.

In contrast to paint, which flakes and peels,
stains "erode," or wear away. Stains must be
reapplied more often than paint, but it’s
an easier job because there is no need for
extensive scraping and sanding. Also, weathering
stain is less of an eyesore than failing paint.

Sheltered porches can be painted with porch and
deck enamel. First, treat the deck with a
water-repellent preservative (check the label to
make sure the product contains a preservative and
is paintable). Second, prime the wood with enamel
diluted with paint thinner. Last, apply two
topcoats of straight enamel. Railings, whether
exposed or sheltered, can be painted with latex
paints.

Remember, wood to be painted should be dry but
not exposed to more than a few weeks of sunlight.
The sun’s ultraviolet rays damage wood
fibers and weaken the wood’s ability to
hold paint.

Mark Knaebe is a research chemist at the
USDA Forest Products Laboratory in Madison,
Wis.